During laser-cutting, bulges of melted and resolidifying metal or metal oxide are formed in the area surrounding the piercing hole when piercing thick workpieces, the dimensions of said bulges increasing significantly with the thickness of the sheet metal. These adhesions around the piercing hole are an impediment, particularly with inner cutting geometries and small components, where the cutting contour necessarily runs along very close to the piercing site. Moreover, the distance sensor system of the laser machining head detects the bulges as an interfering contour and controls the movement of the laser machining head away from the workpiece, giving rise to processing problems. In order to avoid these problems, it is necessary to reduce or prevent bulges adhering to the workpiece.
Japanese patent publication JP05057469A discloses a piercing method in which a wide, shallow trough is produced in the surface or the workpiece in a first processing step with a large distance between the process gas nozzle on the machining head and the workpiece surface. A pierced hole is then produced with a smaller distance between the nozzle and the workpiece surface, a lower laser power and lower process gas pressure.
German patent publication DE102012014323A1 discloses producing, during piercing, a wide cylindrical hole in the upper half of the workpiece in a first process step with a large distance between the process gas nozzle and the workpiece surface. In a second process step disclosed in DE102012014323A1, the laser beam is then focused on the bottom surface of the wide hole, in order to produce a fully pierced hole. During this second step, the process gas is only directed into the wide hole and not at the area surrounding the hole.